This invention pertains to a detector array and method for detecting charged particles and in accordance with one embodiment pertains to such a detector array and method for use with microchannel array plates for photon counting.
One of the important applications of charged particle detector arrays has been in connection with detector arrays for photon counting. In the prior art, instruments for photometric studies such as at ultraviolet and x-ray wavelengths have been either photographic or photoelectric. Photographic instruments, employing film as the detection system, have the great advantage of an image-storing capability. It is therefore possible to use this type of instrument to record a very large amount of data with a single exposure. However, the sensitivity of photographic film is considerably lower than that of a photoelectric detector and the quantum efficiency is typically less than a photoelectric detector. Further disadvantages include the fact that the photographic film response is non-linear as a function of the incident energy, and since the output signal is not electrical in character the photographic film must be recovered, thus inhibiting to some extent its use on orbiting satellites.
Photoelectric instruments, on the other hand, are more sensitive and have a greater stability of response and provide a linear output as a function of the incident energy. However, since most photoelectric detectors do not have image-recording capabilities, the data must be readout sequentially, point-by-point. Consequently, the overall speed of the system is quite low.
The development of the channel electron multiplier and its miniaturization into the microchannel array plate have been important developments in the field of photometrics, combining the advantages of both the photographic and the photoelectric detection systems. The microchannel array plate can be operated as an image-intensifier and has the potential of being developed to yield signal outputs superior to those of conventional photomultipliers. In particular, the microchannel array plate has a photo-counting capability and a negligible dark count rate. These devices can operate stably and efficiently at extreme ultraviolet (EUV) and soft x-ray wavelengths in a windowless configuration or can be installed with a photocathode in a sealed tube for use at ultraviolet and visible wavelengths.
The readout systems generally employed with microchannel array plates in the prior art have been a visible-light phospor coupled to either a vidicon tube or photographic film. In this arrangement, the detected photon is converted to a pulse of electrons in the microchannels; these electrons are accelerated towards the phospor and reconverted to visible photons, which are detected by either the vidicon photocathode or the photographic emulsion. Although the microchannel array plate can provide a gain on the order of 10.sup.7, this system is cumbersome and has all the inherent disadvantages of either the photographic plate or the vidicon tube.
In order to exploit the full sensitivity, dynamic range and photometric stability of the microchannel array plate, it is necessary to employ pulse-counting readout systems working directly at the output of the plate. Some examples of pulse-counting systems to readout spatial information from microchannel array plates have been described in the prior art, but have been designed to employ a limited number of amplifiers, two for a one-dimensional array and four for a two-dimensional array, and have consequently been limited in terms of dynamic range and spatial resolution. This is especially the case for applications at high signal levels such as from laboratory EUV and soft x-ray sources or from telescopes for solar studies at EUV and soft x-ray wavelengths. There have been suggestions of a multielement anode array in the prior art, such as in "The Multianode Photomultiplier", by Catchpole and Johnson, Pub. Astron. Soc. Pacific, volume 84, February 1972, pages 134-136. This article discloses a detector array for use with a microchannel array plate in which a two-dimensional array of individual anode elements (10.times.10 anode array) is provided. Individual amplifier and electronic means appear to be contemplated for each of the individual anode elements. That article further indicates that an alternative readout possibility was contemplated, in which a resistive strip anode was utilized, which is made to act as a voltage divider. Comparison of the pulses at the channel plate and at one end of the resistive anode would enable the position of the pulse to be calculated electronically. This article acknowledges that such a system would have limitations in the maximum pulse rate could handle.
Another version of such a resistive anode readout system for a detector array is described in an article by G.M. Lawrence and E.J. Stone, "Rev. Sci. Instrum. 46, 432" (1975). Such systems have a limited dynamic range and have pincushion distortion inherent in the system.
In applicants' copending application Ser. No. , filed June 8, 1976 and entitled "ONE-DIMENSIONAL PHOTON-COUNTING DETECTOR ARRAY", there is disclosed and claimed a one-dimensional detector array system having good spatial resolution and dynamic range. In that system, the output of a microchannel array plate is proximity focused on a detector array comprising a plurality of parallel, linearly extended anode elements. Individual amplifier and discriminator means are provided for each of the anode elements.